The present invention relates to a vacuum pump, and more particularly to an air bypass system therefor which injects air into a first closed rotor cell of the vacuum pump to minimize cavitation noise.
In a rotary screw oil flooded vacuum pump system, undesirable cavitation noise may be generated when operating near terminal vacuum conditions. Terminal vacuum condition is generally vacuums greater than 27″ Hg at sea level (also known as deep vacuum). The cavitation noise is primary due to torque reversals, which occur in the female rotor when operating in a stated condition.
Torque reversals are a result of oil injected into the rotors and the absence of sufficient air to absorb the compression loads. At terminal vacuum conditions, there is minimal air being compressed and the rotor compression chamber fills with oil. The oil, being incompressible, causes a pressure spike, which reverses the load on the female rotor. The torque reversals are periodic in nature and occur with each rotation of each rotor lobe. The result is rotor vibration, which causes a hammering or cavitation type sound. In addition to the undesirable cavitation noise generation, operation under such conditions for an extended period of time may result in rotor damage.
Conventional vacuum pumps minimize undesirable cavitation noise generation at the terminal vacuum condition by utilization of a vacuum breaker valve to add atmospheric air to the pump intake or by a flow control valve that temporarily reduces an oil flow rate. Although effective, these arrangements may have deleterious effect on the vacuum pump system operation. The vacuum breaker valve reduces the vacuum capability of the pump to the setting of the vacuum breaker. In addition, an air filter must be used with the vacuum breaker to minimize contamination introduction into the pump. If the filter is not properly maintained, airflow may gradually decrease until the cavitation noise reoccurs. Utilization of a vacuum breaker valve also prevents operation at the terminal vacuum capability. Alternatively, reducing oil flow at deep vacuum conditions by a flow control valve increases the operating temperature of the vacuum pump. During reduced oil flow conditions, the oil cooling system provides less system cooling and the operating temperature may approach levels that are detrimental to service life.
Accordingly, it is desirable to provide a rotary screw vacuum pump system, which operates at terminal vacuum capability while minimizing undesirable cavitation noise.